Re: Speeding up a query

----- Original Message -----
From: Andrew Perrin - Demography <aperrin(at)demog(dot)berkeley(dot)edu>
To: <pgsql-sql(at)postgresql(dot)org>
Sent: Friday, April 07, 2000 8:12 PM
Subject: [SQL] Speeding up a query

>
> select last_name from personal_data where id in (
> select distinct id from event_detail, event_universe where
> event_detail.eventtypeid = event_universe.eventtypeid and
> event_universe.studentstatus = 'Student' and
> event_detail.date_effective <= datetime(date('1-jan-1998')))
> and
> id not in (
> select distinct id from event_detail, event_universe where
> event_detail.eventtypeid = event_universe.eventtypeid and
> event_universe.studentstatus = 'NotStudent' and
> event_detail.date_effective <= datetime(date('1-jun-1998')))

Assuming you have indices set up on all appropriate fields, and that you
replace = by LIKE for text fields (don't know if that makes any speed
difference, but it makes it standard SQL), then you need to modify your
query to use EXISTS instead of IN. NOT IN can be particularly slow. You
don't need the DISTINCT's either, I think.

SELECT last_name FROM personal_data WHERE EXISTS (SELECT id FROM
event_detail,event_universe WHERE id=personal_data.id AND
event_detail.eventtypeid=event_universe.eventtypeid AND
event_universe.studentstatus LIKE 'Student' AND
event_detail.date_effective<=datetime(date('1-jan-1998')))
AND NOT EXISTS
(SELECT id FROM event_detail,event_universe WHERE id=personal_data.id AND
event_detail.eventtypeid=event_universe.eventtypeid AND
event_universe.studentstatus LIKE 'NotStudent' AND
event_detail.date_effective<=datetime(date('1-jun-1998')));

Should be right...
M.